Hand-actuated gimlets are known which enable the providing of lead holes, for example in wood. Likewise known, for piercing the actual hole, are drill bits or twist drills including one or several peripheral undercuts generally in the form of a helix and which begin at the penetrating end of the tool. Such peripheral undercuts, of a generally helical form hereinafter referred to as flutes, serve, on the one hand, to evacuate the chips removed by the cutting portions of the tool. On the other hand when the tool is at least partially conical they serve to enlarge the hole or lead hole by participating in the removal of material by means of their cutting profile. The experienced user of a drill bit takes care to restrain the advance of his tool in order to avoid the undesirable phenomenon of screwing in, by which the advance of the tool is determined simply by the speed of rotation and the pitch of the helix. A perceptible start of screwing in must be corrected by a stopping or reversal of the advance and by slowing, stopping or reversal of the rotation if the tool shows a tendancy to block. Blocking can cause breaking of the tool.
The screwing-in tendency becomes that much stronger to the extent that the tool exhibits a generally conical rather than cylindrical form.
In the special case of dental surgery, the boring of root canals must exhibit a conical profile in view of the filling which will follow. Consequently the tools used have such a strong tendency to screwing in that they are used above all for manually performed axial reaming. Their manipulation is delicate since breaking such a tool can be very detrimental.
Canal boring with continuous driving of the instruments in rotation at low speed is only recently recommended. Such procdure has as principal advantage that of not deforming the natural path of a tooth canal whilst enlarging it.
In such technique it has been attempted to eliminate the screwing-in tendency by blunting the cutting angles of the flutes. But not only is the efficiency of material removal diminished along with undesirable heating but there subsists a certain screwing-in tendency.